Tilted body or tilted world? A field study of verticality misperception on a moving mountain tram

Abstract

We used a tilt illusion experienced on the Hong Kong Peak Tram to study the multimodal nature of verticality judgment. When passengers sitting in the reclining mountain tram, the skyscrapers along the tram way appear to tilt as much as 30° towards the mountain. We conducted two field experiments on the mountain tram to investigate the origin of this verticality illusion. In study 1, we isolated the contribution from the vestibular, tactile, proprioceptive, and visual systems and found that the signals from any sensory system alone did not affect the perceived tilt much. In contrast, the illusion could be nearly abolished when the (erroneous signals from all the) sensory systems were simultaneously compensated for, implying the multiplicative multi-sensory nature of the tilt illusion. In study 2, we asked blindfolded observers to report their haptic subjective vertical (SV) and found increased SV errors with increasing slope. This differed from laboratory studies documenting superior verticality judgment regardless of body orientation, possibly because of motion, an often neglected factor in verticality perception. The significantly smaller SV error than illusion size implies that the tilt misperception is not purely a result of the miscalibration of the SV. The study was supported by grants from the Hong Kong Grant Research Council and the University of Hong Kong Seed Funding Programme for Basic Research to Chia-huei Tseng, and by awards from the Serena Yang Educational Fund and the Deutscher Akademischer Austauschdienst (German Academic Exchange Council) and National Science Council of Taiwan to Lothar Spillmann.